1. Field of the Invention
The invention relates to circuit breakers having contact arms which are rapidly blown open in response to a short circuit before the spring driven operating mechanism can respond to the fault. More particularly, it relates to a catch mechanism which latches the contact arm in the blow open position and automatically releases as the operating mechanism responds to the overcurrent condition.
2. Background Information
Circuit breakers have sets of separable contacts which include a fixed contact and a movable contact mounted on a movable contact arm. The movable contact arm is rotated by an operating mechanism between a closed position in which the movable contact engages the fixed contact, and an open position in which the contacts are separated to interrupt current flow. Typically, the operating mechanism is spring powered to rapidly open the contacts in response to an overload condition.
The response of the typical spring driven operating mechanism is relatively slow. It is known to provide such circuit breakers with a blow open feature which utilizes the strong magnetic fields generated by very large overcurrent conditions such as accompany a short circuit to quickly open the contacts faster than the response time of the operating mechanism. Typically, in such circuit breakers with a blow open feature, the fixed conductor to which the fixed contact is secured, is positioned adjacent the movable contact arm with the contact closed to carry current in a direction opposite to the current through the movable contact arm. This generates magnetic repulsion forces tending to separate the contacts. Under normal current conditions and moderate overcurrent conditions, the contacts are held in a closed position by contact springs. However, the repulsion forces generated by a short circuit current are so high that they overcome the contact spring forces and rapidly blow the contacts open.
An example of a circuit breaker incorporating a blow open feature is found in U.S. Pat. No. 5,341,191. In this circuit breaker, the movable contact arm is mounted by a two-part pivot assembly. The pivot assembly includes a main carrier which is pivotally rotated by the operating mechanism to open and close the contacts. The movable contact arm is mounted on a second carrier which is pivotally mounted on the first carrier. Normally, the operating mechanism rotates the main carrier which carries with it the secondary carrier and the main contact arm to open and close the separable contacts. In response to a short circuit current, the secondary carrier is rotated relative to the main carrier in response to the very high magnetic repulsion forces generated by the fault current. The current required for the contact arm to blow open is determined by a spring loaded cam assembly which includes cam surfaces on the secondary carrier and spring biased cam followers mounted on the main carrier.
While the blow open circuit breaker of U.S. Pat. No. 5,341,191 has been effective in responding rapidly to short circuit currents, the very large repulsion forces accompanying a short circuit generate a great deal of kinetic energy in the moveable contact arm which, when it encounters stops at the blow open position rebounds towards the fixed contact. This can result in restriking of an arc between the contacts necessitating a second extinguishing of an arc which imposes heavy wear on the contacts. One solution has been to absorb the kinetic energy of the moveable contact arm by mounting dead rubber stops in the cover which the arm strikes, but often the energy cannot be absorbed in this manner.
U.S. patent application Ser. No. 08/806,030, filed on Sep. 3, 1996, addresses this problem by providing a latch which latches the contact arm in the blow open position. In a preferred embodiment of the invention this latch takes the form of a cantilevered leaf spring forming a detent which is engaged by the secondary carrier as the moveable contact arm blows open. When the operating mechanism finally responds to the short circuit and rotates the main carrier to the open position, the secondary carrier is disengaged from the leaf spring. In a second embodiment of the invention described in this patent application, there is a cam and cam follower arrangement between the main carrier and the secondary carrier. The latch means is a notch in the cam which is engaged by a spring biased cam follower to latch the moveable contact arm in the blow open position. In this embodiment, the operating mechanism must overcome the spring bias of the cam follower to disengage the latch as the operating mechanism responds to the short circuit current. The latch mechanism described in U.S. patent application Ser. No. 08/806,030 is not adaptable to all circuit breakers with blow open contact arms.
There is a need therefore for an improved latch for latching a moveable contact arm of a circuit breaker in the blow open position.
There is a further need for a circuit breaker incorporating such an improved latch.
There is an additional need for such a circuit breaker in which the latch provides a positive mechanical latching of the contact arm in the blow open position but which is automatically released as the operating mechanism responds to the short circuit.
There is a still further need for such a circuit breaker in which the mechanism latch does not interfere with normal operation of the circuit breaker when the moveable contact arm is not blown open but is rotated to interrupt current by the operating mechanism.